1. Field of the Invention
The present invention relates to a display control circuit, a display control method and a liquid crystal display device, which are suitable for moving-image display using a liquid crystal display panel of, e.g. an OCB (Optically Compensated Birefringence).
2. Description of the Related Art
Flat-panel display devices, which are typified by liquid crystal display devices, have widely been used as display devices for computers, car navigation systems, TV receivers, etc.
The liquid crystal display device generally includes a liquid crystal display panel including a matrix array of liquid crystal pixels, a backlight that illuminates the liquid crystal display panel, and a display panel control circuit that controls the display panel and the backlight. The liquid crystal display panel is configured such that a liquid crystal layer is held between an array substrate and a counter substrate.
The array substrate includes a plurality of pixel electrodes that are arrayed substantially in a matrix, a plurality of gate lines that are arranged along rows of the pixel electrodes, a plurality of source lines that are arranged along columns of the pixel electrodes, and a plurality of switching elements that are disposed near intersections between the gate lines and the source lines. Each of the switching elements is formed of, e.g. a thin-film transistor (TFT). When one associated gate line is driven, the TFT is turned on to apply a potential of one associated source line to one associated pixel electrode. The counter substrate is provided with a common electrode that is opposed to the pixel electrodes arrayed on the array substrate. Each pair of pixel electrode and common electrode, together with a pixel area that is a part of the liquid crystal layer situated between these electrodes, constitute a pixel. In the pixel area, the alignment of liquid crystal molecules is controlled by an electric field that is created between the pixel electrode and the common electrode. The display control circuit includes a gate driver that drives the gate lines, a source driver that drives the source lines, and a controller circuit that controls the gate driver, the source driver and the backlight.
In the case where the liquid crystal display device is used for a TV receiver that principally displays a moving image, it is proposed to use a liquid crystal display panel of an OCB-mode, in which liquid crystal molecules exhibit good responsivity (see Jpn. Pat. Appln. KOKAI Publication No. 2002-202491). In this liquid crystal display panel, the liquid crystal molecules are aligned in a substantially horizontal splay alignment prior to supply of power by alignment films that are provided on the pixel electrode and common electrode and are rubbed in directions parallel to each other. In the liquid crystal display panel, a display operation is performed after the splay alignment of the liquid crystal molecules is transferred to a bend alignment by a relatively strong electric field that is applied in an initializing process following supply of power.
The reason why the liquid crystal molecules are aligned in the splay alignment before supply of power is that the splay alignment is more stable than the bend alignment in terms of energy in a state where the liquid crystal driving voltage is not applied. As a characteristic of the liquid crystal molecules, the bend alignment tends to be inverse-transferred to the splay alignment if a state where no voltage is applied or a state where a voltage lower than a level at which the energy of splay alignment is balanced with the energy of bend alignment is applied, continues for a long time. The viewing angle characteristic of the splay alignment significantly differs from that of the bend alignment. Thus, a normal display is not attained in this splay alignment.
In a conventional driving method that prevents the inverse transfer from the bend alignment to the splay alignment, a high voltage is applied to the liquid crystal molecules in a part of a frame period for a display of a 1-frame image, for example. This high voltage corresponds to a pixel voltage for black display in an OCB-mode liquid crystal display panel, which is a normally-white type, so this driving method is called “black insertion driving.”
A pixel voltage for black insertion and a pixel voltage for gradation display are applied to all liquid crystal pixels on a row-by-row basis in one frame period, i.e. one vertical scanning period (V). The ratio of a holding period of the pixel voltage for black insertion to a holding period of the pixel voltage for gradation display is a black insertion ratio. In a case where each gate line is driven for black insertion in a half of one horizontal scanning period (H), i.e. H/2 period, and is driven for gradation display in a subsequent H/2 period, the vertical scanning speed becomes twice higher than in the case where black insertion is not executed. Since the value of the pixel voltage for black insertion is common to all pixels, it is possible to drive, for instance, two gate lines together as a set. In a case where two gate lines of each set are driven together for black insertion in a 2H/3 period, and are sequentially driven for gradation display in a 4H/3 period (2H/3 for each of two gate lines), the vertical scanning speed becomes 1.5 times higher than in the case where black insertion is not executed.
In the meantime, since the liquid crystal display panel is a hold-type display device that holds a display state until updating of image data, it is difficult to smoothly display the motion of an object, owing to the effect of retinal persistence occurring on a viewer's vision in moving-image display. In the black insertion driving, the retinal persistence is cleared by a discrete pseudo-impulse response waveform of pixel luminance. Thus, the black insertion driving is effective in improving the moving-image visibility, which lowers due to the viewer's vision. However, the black display state that is obtained by the black insertion driving is not the perfect black, which would be obtained, for example, when the backlight is turned off. Under the circumstances, it is considered to obtain good moving-image visibility by making use of a blinking driving in which the backlight is blinked. A black insertion ratio for preventing inverse transfer is about 25%. As the black insertion ratio is increased, the moving-image visibility is improved accordingly.
In the case of blink-driving the backlight, the duty ratio of the backlight, that is, the ratio of a turn-on period to a blinking cycle that is normally one vertical scanning period, can be used in order to adjust the brightness of the entire liquid crystal display panel on the basis of a dimmer signal that is supplied from an external signal source. If the duty ratio of the backlight is 100% that means continuous turn-on, it is not expectable that the moving-image visibility is improved by the blink-driving. If the duty ratio is set at, e.g. 70% or 50%, it is possible to decrease the brightness of the entire liquid crystal display panel, while improving the moving-image visibility.
However, even if the duty ratio of the backlight is varied by the dimmer signal, the backlight is turned off during the holding period of the pixel voltage for gradation display or turned on during the holding period of the pixel voltage for black insertion, owing to the effect of the black insertion ratio and black insertion timing on the liquid crystal display panel side, and the brightness of the liquid crystal display panel does not linearly vary in relation to the variation in duty ratio of the backlight. In order to solve this problem, it may be possible to non-linearly vary a dimmer signal from an external signal source. In this case, however, the provision of the external signal source is a burden on the user, and a very complicated control is required.